/*
 *  Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF licenses this file to You under the Apache License, Version 2.0
 *  (the "License"); you may not use this file except in compliance with
 *  the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */
package org.apache.commons.collections.comparators;

import java.io.Serializable;
import java.util.Comparator;

/**
 * Reverses the order of another comparator by reversing the arguments to its
 * {@link #compare(Object, Object) compare} method.
 *
 * @since Commons Collections 2.0
 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr
 * 2008) $
 *
 * @author Henri Yandell
 * @author Michael A. Smith
 *
 * @see java.util.Collections#reverseOrder()
 */
public class ReverseComparator implements Comparator, Serializable {

	/**
	 * Serialization version from Collections 2.0.
	 */
	private static final long serialVersionUID = 2858887242028539265L;
	/**
	 * The comparator being decorated.
	 */
	private Comparator comparator;

	/**
	 * Creates a comparator that compares objects based on the inverse of their
	 * natural ordering. Using this Constructor will create a ReverseComparator
	 * that is functionally identical to the Comparator returned by
	 * java.util.Collections.<b>reverseOrder()</b>.
	 *
	 * @see java.util.Collections#reverseOrder()
	 */
	public ReverseComparator() {
		this(null);
	}

	/**
	 * Creates a comparator that inverts the comparison of the given comparator.
	 * If you pass in
	 * <code>null</code>, the ReverseComparator defaults to reversing the
	 * natural order, as per
	 * {@link java.util.Collections#reverseOrder()}</b>.
	 *
	 * @param comparator Comparator to reverse
	 */
	public ReverseComparator(Comparator comparator) {
		if (comparator != null) {
			this.comparator = comparator;
		} else {
			this.comparator = ComparableComparator.getInstance();
		}
	}

	/**
	 * Compares two objects in reverse order.
	 *
	 * @param obj1 the first object to compare
	 * @param obj2 the second object to compare
	 * @return negative if obj1 is less, positive if greater, zero if equal
	 */
	public int compare(Object obj1, Object obj2) {
		return comparator.compare(obj2, obj1);
	}

	/**
	 * Implement a hash code for this comparator that is consistent with
	 * {@link #equals(Object) equals}.
	 *
	 * @return a suitable hash code
	 * @since Commons Collections 3.0
	 */
	@Override
	public int hashCode() {
		return "ReverseComparator".hashCode() ^ comparator.hashCode();
	}

	/**
	 * Returns
	 * <code>true</code> iff <i>that</i> Object is is a
	 * {@link Comparator} whose ordering is known to be equivalent to mine. <p>
	 * This implementation returns
	 * <code>true</code> iff
	 * <code><i>object</i>.{@link Object#getClass() getClass()}</code> equals
	 * <code>this.getClass()</code>, and the underlying comparators are equal.
	 * Subclasses may want to override this behavior to remain consistent with
	 * the {@link Comparator#equals(Object) equals} contract.
	 *
	 * @param object the object to compare to
	 * @return true if equal
	 * @since Commons Collections 3.0
	 */
	@Override
	public boolean equals(Object object) {
		if (this == object) {
			return true;
		} else if (null == object) {
			return false;
		} else if (object.getClass().equals(this.getClass())) {
			ReverseComparator thatrc = (ReverseComparator) object;
			return comparator.equals(thatrc.comparator);
		} else {
			return false;
		}
	}
}
